But no clinically significant differences were seen between antidepressants and placebo in mildly depressed patients nor in the majority of patients with severe depression.

Only in some of the most severely depressed patients did the antidepressants show any benefit over the placebo.

However, the researchers say this may be due to a decrease in response to the placebo drug rather than an increased response to the antidepressant.

What do the researchers say?Lead researcher Professor Irving Kirsh, from the department of psychology at the University of Hull, said: 'The difference in improvement between patients taking placebos and those taking antidepressants is not very great.

'This means that depressed people can improve without chemical treatments.'

'There seems little reason to prescribe antidepressant medication to any but the most severely depressed patients, unless alternative treatments have failed to provide a benefit.'

What do other experts say?Dr Tim Saunders, GP and mental health lead for Chester West PCT, said: 'There is no need for a knee-jerk reaction to one study on antidepressants. I would advise GPs to continue to prescribe antidepressants in line with NICE guidance.'

NICE guidelines, issued in May 2007, state that antidepressants are prescribed to people with moderate or severe depression who have already been offered computerised cognitive behavioural therapy or psychological interventions.

Dr Chris Manning, chief executive of the charity Primary Care Mental Health and Education, said: 'I am convinced that there is a population of people with depression who will continue to require antidepressants.'

But Dr Ian Walton, a GPSI in psychiatry in Sandwell, West Midlands, said antidepressants had not been as effective as hoped despite a doubling of prescriptions in 10 years.

'We are piloting alternatives to antidepressants such as psychological therapy and positive mental training at our practice,' he said. 'It is important to help patients tackle the underlying causes of their depression.'

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